The development of peptides as therapeutic agents has considerable potential and much effort is being expended, worldwide on this goal. A major challenge, however, is the delivery of peptides to their site of action because of the impermeability of many biological membranes and also rapid drug elimination, often involving the liver. These factors have the potential to be rate-limiting after oral dosing - the most convenient and widely route of administration; nevertheless, orally effective peptides have been developed. the overall objective and long term goal of the proposed research is to determine the mechanisms by which such oligopeptides are absorbed from the gastro-intestinal tract and undergo first-pass elimination involving hepatic uptake and secretion unchanged into bile. Such knowledge will be critical if orally available peptides are to be developed in a rational fashion. the working hypothesis is that carrier-mediated processes are required for translocation. Accordingly, the proposed research is designed to identify and characterize the carrier systems for oligopeptides at the brush-border and basolateral membranes of the intestine and the sinusoidal and canalicular membranes of the liver. The hexapeptide, ditekiren, an orally effective renin inhibitor that is completely eliminated from the body by biliary excretion of unchanged drug, will be used as a model drug to probe these processes in the rat. A variety of complementary experimental approaches will be employed to systematically investigate the transport processes. In the liver, 3H-ditekiren uptake will initially be studied in isolated hepatocytes and insights obtained on critical determinants related to the bioenergetics of transport. This data will also provide preliminary information on the possible role of known bile acid and other substrate carrier systems in oligopeptide uptake. subsequent studies will use purified hepatic sinusoidal and canalicular vesicle membrane preparations to further define and characterize the driving forces for transport. It is also proposed to investigate oligopeptide transport by individual carrier proteins known to be present in the sinusoidal and canalicular membranes following their functional expression in Xenopus laevis oocytes. Oligopeptide transport across the gastro-intestinal epithelium will be studied in an analogous fashion using brush-border and basolateral membrane vesicles obtained from the jejunum and ileum, which are known to be the locations of the di/tripeptide and bile acid carriers, respectively.